During many clinical procedures, a physician requires the reduction or complete stoppage of blood flow to a target region of the patient's body to achieve therapeutic benefit. A variety of devices are available to provide occlusion of blood vasculature including embolic coils, metal-mesh vascular plugs, beads, particles and glues. Interventional radiologists and vascular surgeons (and similar medical specialists) draw from these therapeutic options based upon the specific need and confidence of a rapid and effective occlusion given the attributes and deficiencies of each of these options. These devices may be used to occlude vasculature in situations requiring treatment, for example, of arteriovenous malformations (AVMs), traumatic hemorrhage, fistulae, some aneurysm repair, uterine fibroid, and tumor embolization. For these clinical treatments, the blood flow through a target section of a blood vessel, aneurysm or defect must be stopped. The device is introduced into the blood vessel through a sterile delivery catheter or sheath using common percutaneous access outside the body. The delivered, artificial device induces an initial reduction of blood flow through a simple mechanical blockage which in turn triggers the body's natural clotting process to form a more complete blockage comprised of the thrombus adhered to the device.
One specific clinical purpose is to fill an aneurysm space, or sack, that resides behind an endograft for repair of Abdominal Aortic Aneurysms. The endograft is intended to isolate a weakened vessel wall in the aorta from blood pressure and thereby reduce the risk of rupture. While the graft may successfully isolate the aortic blood flood, side branches and feeders may connect into the aneurysm sack and continue to present blood pressure on the weakened vessel wall. One attempt for resolution is to access this sack behind the endograft and fill this space with embolic coils. Access may be performed through a catheter, trocar or needle cannula, the latter may be through tissue by puncturing the aneurysmal wall. As this space can be relatively large, independent coils of defined length can only contribute a small percentage of displacement. In order to fill this space, a very large number of metallic coils may be used resulting in a very large metal mass to reduce blood flow and ultimately achieve flow stasis in the sack behind the graft. This is very costly, requires considerable x-ray exposure to both physician and patient, and the resulting metal mass can detrimentally affect post procedure patient imaging with either CT or MR scanning.
Current embolic coils are made from biocompatible materials and provide a biodurable, stable blockage of blood flow. The coils anchor to the vessel wall or aneurysm through radial compliance pressing onto the vessel wall surface. Coils must be suitably anchored to avoid migrating downstream under the forces of the blood flow, which can be significant in larger vasculature. Embolic coils are often shaped for flexibility through a primary coiling and for achieving a “coil pack” within the vessel through a secondary, sometimes complex, three dimensional shape. The coil pack appears as a relatively random crossing and intertwining of the coil within the vessel. After slowing the blood flow, over time, a clot forms around the embolic coil and blood flow through the section is completely blocked.
Typical embolic coils are formed by two major steps: 1) a wire of platinum or other bio-compatible material is wound into a spring, forming what is commonly referred to as a primary coil; and 2) the primary coil is in turn wound around a mandrel having a more complex shape and is subject to high heat to yield a secondary coil. The secondary coil is thus a coiled wire of complex shape or, if helical, a larger curl diameter. Coils can also be provided in multiple secondary shapes including multiple helical curl diameters and in tapered helical shapes with one end employing a large curl diameter and the other end a small curl diameter. These metal coils are straightened, within their elastic bending limit, so as to be advanced into a delivery catheter and pushed down the catheter by a guide wire, pusher, or a detachable pre-attached pusher, until expelled into the vessel. Often, polymeric fibers are applied to the metallic coils in order to increase a thrombus response in addition to providing a scaffolding for thrombus to adhere to and be retained on the coil.
Embolic coils are sized to fit within the inner lumen of a catheter or sheath to be delivered to the target occlusion site individually and sequentially. Typically, a physician will use multiple coils of discrete lengths to occlude a single vessel and, in some cases, especially for larger blood vessels (above 5 mm or so), the physician may use a significant number of coils to achieve cessation of blood flow. To complete an occlusion procedure with embolic coils, the physician must sequentially reload the catheter with several individual coils until he/she has determined the occlusion is sufficient. The physician typically determines whether sufficient coils have been deployed by assessing the level of occlusion of the vessel flow by using contrast media in concert with typical medical imaging techniques. This “place and assess” method can extend the medical procedure time, expose the patient to increased levels of contrast agent, and expose both the patient and the physician to increased radiation through extensive imaging.
Embolic coils are also known for challenges in achieving precise vascular placement. Many of these coils are simply pushed out of the end of a delivery catheter. The final coil pack location is dependent upon whether the coil has been properly sized before deployment and whether the coil was properly anchored into a side vessel/branch as prescribed by several of the coil manufacturers for greater confidence in the final position of the coil packs. Both of these techniques require a high level of physician skill if there is a desire to accurately position both the distal and proximal faces of the coil pack in a vessel using sequential, pushable coils. Some of the coil manufacturers provide a detachable coil—a device that encompasses a coil of discrete length, removably attached to a second delivery system or control wire. At the physician's discretion a placed coil can be released from a delivery control wire. If the coil is not in the proper location it can be retracted and replaced if needed to achieve better position before release. Only the proximal end of the coil is attached to the control wire, resulting in only indirect control of the position of the coil pack's distal face.
Using coils for embolization can present other unique challenges. Voids in the coil pack, developed either during the procedure or post-operatively, can cause channels and resulting blood flow in an unintended area. This condition is typically referred to as recanalization. Depending upon the significance of the condition (e.g., internal hemorrhage), retreatment or surgical intervention may be necessary. The sequential use of independent coils of fixed lengths can be a very time consuming procedure where the intended target is a large vessel. An intraoperative outcome may appear stable and occluded, but greater certainty could be achieved by placing one or more additional coils. However, the challenges of deploying one additional coil to further increase the coil pack density may not be deemed desirable given the coil cost and time involved with placement. The ability to quickly and reliably develop a consistently dense coil pack in a vessel is an important characteristic of a successful vascular occlusion product or aneurysm filling device.
In addition, independent embolic coils can be easily misplaced. Embolic coils may either be injected through a delivery catheter with a syringe filled with saline, pushed by an independent guide wire, or deployed with a detachable pusher that is only connected to the coil via its proximal end. The coil pack shape is dependent upon the successful placement of the initial coil and the ability to engage the subsequent coils in an intermixed and tangled mass of high density. Accordingly, coils can easily be misplaced should the initial coil not land correctly or be slightly undersized to the target vessel and slip beyond the target location. As such, embolic coil packs are known for a high propensity of being elongated in overall size. While these devices have been employed clinically for years and the technique is generally accepted, coils present significant challenges when attempting to embolize in a very precise or limited section of vasculature.
Metal mesh vascular plug devices have also been developed and commercialized to achieve vascular occlusion. These devices achieve occlusion with a single deployment using a metal mesh to provide mechanical flow blockage and, after some time, thrombus forms and a complete occlusion results. When deployed, these devices assume the form of metal mesh balloons or baskets, with one or more lobes contacting the vascular wall, but with defined proximal and distal faces. With occlusion occurring after a single device deployment, these products address many of the deficiencies of embolic coils. However, due to the porosity of the mesh basket and the lack of the polymeric fibers used in coils, the metal mesh plugs have been shown to take longer to achieve occlusion than a properly placed embolic coil pack. Further, the fixed shape of these devices makes them unattractive for use in odd-shaped spaces such as an aneurysm sack that occurs behind an endograft stent.
Further, these metal mesh devices are relatively stiff due to their construction and have limited ability to traverse sharp turns found in catheters that have been placed in a highly tortuous vascular path. The mesh is collapsed into a narrow tube-like shape for introduction and deployment through a delivery catheter or sheath before expanding into the balloon like shape upon deployment. This narrow tube-like shape allows the device to be delivered in the central lumen of small catheters or sheaths similar to coils. However, when the mesh is collapsed, it elongates and becomes a fairly rigid tubular structure. Thus, while being capable of entry into a small delivery catheter, metal mesh devices have limited ability to traverse sharp turns found in catheters that have been placed in a highly tortuous path to reach the target vessel for occlusion. Subsequently, the advantages of a single occlusion device are offset by the slow and incomplete occlusion performance and the limited application to occlusion target sites that are less tortuous to access.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded subject matter by which the scope of invention is to be bound.